Multiple drug resistant tuberculosis (MDRTB) has recently emerged as a serious public health concern. To develop better drugs to treat MDRTB and design improved diagnostic protocols, it is imperative that drug targets and drug resistance mechanisms of M. tuberculosis are defined. Our laboratory has been investigating the mechanisms of resistance to INH and SM in single and multiple drug resistant strains of tuberculosis. We have demonstrated that reduced sensitivity to INH is often associated with deletions or mutations in the catalase-peroxidase gene (kat G). One of seven INHr mutants examined thus far has a kat G gene deletion. Three of the other INHr strains have an identical mutation in the kat G coding sequence at a site which may effect INH binding by the catalase-peroxidase. Using heterodaplizanalysis we are currently examining whether another mutational events in the kat G gene of INHr strains may be associated with drug resistance. Furthermore, we have shown that one mechanism of resistance to SM involves a mutation in the gene encoding the ribosomal S12 protein (rpsl). Six singly SMr M. tuberculosis strains have an identical point mutation at codon 43 of the rps L tene. This same mutation confuse SM resistance in E. coli.